This invention is related to the field of knock detection relating to an internal combustion engine, and more specifically to a system for determining a knock condition while measuring spectral energy sensed by an engine coupled knock spectra responsive sensor.
Engine control systems with knock detection capability, are used to detect and eliminate knocking conditions, characteristic of the operation of internal combustion engines. Eliminating a knocking condition is important because, left unchecked engine power and efficiency will suffer, and combustion chamber and spark plugs will be damaged.
Knock detection systems typically use an accelerometer based sensor mounted on an engine for converting engine vibration into an electrical signal. This electrical signal includes a broad spectrum of information about the engine""s operating condition. Included in this signal is the knock information, and extraneous information known as noise. When knocking occurs, the knock component of the signal results from an initial shock wave followed by gas oscillations in the combustion chamber during combustion. The noise component of the signal generally is significant and can be large enough to mask the knock component. The noise component may be comprised of several sources including piston slap vibration, valve closing vibration, and other systemic noise.
Various techniques have been applied to extract the knock information from the electrical signal representative of engine vibration, however, these techniques have been limited in accuracy and reliability. The knock component of the signal occurs at several different frequencies and the energy level shifts in emphasis between those frequencies. This partially is related to, gas temperature, and combustion chamber geometry. Moreover, there are variations from engine to engine and from cylinder to cylinder within the same engine that affect the relationship of the knock component to the noise component.
To provide for this range of conditions typical knock detection schemes use a broadband bandpass filter to extract knock information from the broadband electrical signal. Some schemes include engine crank angle based windowing, to restrict the analysis of knock to when it could likely occur in the combustion cycle. Other schemes attempt to improve the signal-to-noise ratio of the measurement by employing a second, or noise, bandpass filter having a passband located distant to the passband of the knock bandpass filter. The output of this noise bandpass filter is then subtracted from the output of the knock bandpass filter. The concept here is that the noise bandpass filter will indicate the level of noise signal located apart from the knock bandpass filter. This assumes that noise is broadband, and that the noise component in the knock band is of substantially the same magnitude. Thus, by subtracting the output of the noise bandpass filter from the output of the knock bandpass filter a more accurate representation of the knock component may be obtained.
This scheme is defective for several reasons, including that the filters intrinsically have a phase delay error. When the noise is subtracted this error detracts from accurately canceling the noise component of the signal. Further, since the noise band is located apart, only the noise located apart from the characteristic knock frequencies can be detected. In fact, the noise is not really broadband and there can be substantial noise located close to the characteristic knock frequencies that will not be canceled in this scheme. Also, with present schemes, extensive calibration is required to determine for each engine type a characteristic knock frequency, and if used, appropriate crank angle window, and a knock threshold. Additionally, as the engine speed increases, the systemic noise content increases substantially, and current systems perform inadequately.
What is needed is an improved system for detecting knock in internal combustion engines that is more reliable, and accurate, can detect knock at high engine speeds, requires minimum calibration and can be easily applied to different engine families.